1. Field of the Invention
The present invention relates to transmission power control method, communication device and radio communication system, for transmission power control and communication quality control.
2. Related Background Art
For future radio communication systems, the implementation of multimedia services is demanded, so it is indispensable to implement control while being conscious of the demand for quality (QoS: Quality of Service), which differs depending on the application. In particular, the demanded values for various factors, including the reception quality (BER: Bit Error Rate), allowable transmission delay, transmission speed, the maximum count of retransmission of transmission data, and the rate of data not received, differ between real-time applications such as voice communication, and non-real-time applications such as data communication. Also the traffic generation pattern and the transmission data volume are considerably different, and various aspects must be considered. On the other hand, it is also important to support QoS demands as well as using radio resources at high efficiency, since resources in radio communication are limited.
In radio communication, reception quality is not constant, but changes constantly because of such influences as the radical change of the propagation path environment and interference by other signals which use the same frequency band. In order to reduce the influence of the changes of the propagation path and interface at the reception side, the transmission side generally transmits data adding a margin to the minimum transmission power to be required in order to achieve the reception quality demanded at the reception side.
This transmission power margin has been a constant value in prior art, but by changing this margin, depending on the QoS request and the data transmission frequency, the limited resources of radio can be effectively used and QoS can be supported.
An example of prior art focusing on QoS and transmission power is the “Power control method for communication systems”, stated in Japanese Patent Application Laid-Open 2000-91985. This is a method for the CDMA (Code Division Multiple Access) system in FIG. 13A, where the traffic of voice communication and data communication (required bit error rate is different) coexist, wherein the quality of the voice terminal is monitored and the transmission power and the transmission rate of the data terminal is gradually increased without influencing the quality of the voice terminal. However, the power margin is the same for each type of traffic, as shown in FIG. 13B, and no consideration is taken for retransmission. Generally a data terminal which requires a higher speed of transmission without reception errors, more so than a voice terminal, has a higher transmission power, but if a same margin as a voice terminal is provided, a large interference is generated to peripheral mobile stations. Particularly in a CDMA system which uses the same frequency band for all the cells, an increase of interference greatly affects communication quality.
As a prior art focusing on the number of retransmission times of data and the transmission power, “Method for transmission via wireless links”, stated in Japanese Patent Application Laid-Open No. 2001-7764, is a Method based on the number of received ACKs, and is a technology for increasing or decreasing the transmission power level according to the size of the value of (number of lost ACKs)/(Number of transmitted ACKs). With this technology, however, a long control time interval must be taken in order to count the number of ACKs, which makes it difficult to perform strict control with a short control time interval.
“Mobile radio communication system and power control method”, stated in Japanese Patent Application Laid-Open No. H8-167872, on the other hand, discloses a technology for changing the value of the transmission power margin without keeping it constant. This technology, however, only changes the value of the margin according to the size of the fading fluctuation, as shown in FIG. 14, and aims at decreasing the power consumption of a mobile station, for example, by decreasing the margin when the drop in the reception level by fading is small. Therefore with this technology, it is difficult to sufficiently support QoS, a critical element, in the next generation of mobile communication systems.
As described above, in prior art aiming at supporting QoS, the margin to be provided to transmission power is uniform, and a method for changing the transmission power margin according to QoS or the data retransmission frequency has not yet been proposed.
However setting the margin according to the QoS and the data retransmission frequency is a desired technology as one of the control technologies to support QoS and to use radio resources effectively. Particularly for future radio communication systems, support of QoS demanded by users is indispensable, and performing control while considering QoS in not only applications and higher layers but also in physical layers and link layers is critical, so setting the margin according to QoS and the retransmission frequency is highly anticipated.
To solve the above problems, it is an object of the present invention to provide a transmission power control method, communication device and radio communication system, which can support various QoS demands and effectively use radio resources.